1. Field of the Invention
The present invention relates to a tappet for an internal combustion engine, particularly of the kind having a ceramic seat plate for contact with a cam.
2. Description of the Prior Art
A prior art tappet for an internal combustion engine has usually been made of a single metallic material such as chilled cast iron or hardened steel in its entirety.
Recently, in order to meet the demand for a higher output of an engine through improvement of the wear resistance of the tappet, it has been proposed to provide a tappet consisting of a tappet main body made of a single metallic material and a ceramic disk or seat plate brazed to an end of the tappet main body.
The prior art tappet having the ceramic seat plate is encountered by the following problems.
(a) If the tappet main body is made of such a metallic material that deteriorates in mechanical properties due to the heat during brazing, heat treatment of the tappet after brazing is necessitated. However, such heat treatment may lower the strength of the brazed portion. Accordingly, the tappet is desired to be made of such a metallic material that becomes hardened by the heat during brazing. A satisfactory metallic material for this end is considered to be SNCM 630 (nickel-chromium-molybdenum steel according to Japanese Industrial standards), SKC 24 (hollow drill steel according to Japanese Industrial Standards), etc. However, these metallic materials are expensive. Accordingly, when such metallic materials are used for the tappet main body, a high manufacturing cost of the tappet will inevitably result.
(b) A metallic material suitable for being joined with the ceramic seat plate is generally poor in machinability, resulting in a high manufacturing cost. This is particularly true when the tappet needs to be machined for forming a deep hole.